In a Xubuntu team meeting, it was decided that Xubuntu 13.10 Saucy Salamander won't ship with Mir & XMir by default.
The reasons behind the decision is that XMir doesn't bring improvements yet and there are some issues with XMir and Xubuntu for now.
The first question, "Should we wait for improvements landing to XMir later (but before the FF)?" was rejected with 0 votes for waiting, 4 votes against and 1 abstention. The second question was if Mir should be included by default in Xubuntu 13.10 and with 5 votes against, 0 abstentions and 0 votes for including Mir, it was decided that Xubuntu 13.10 Saucy Salamander won't use Mir by default.
XMir will be evaluated for inclusion in Xubuntu during the next cycle "when it has the features we want and is more mature, hopefully at that point it can be a better option for our users".
In the same meeting there was also a discussion about including some Xfce 4.11 components (xfce4-settings which brings improved multi-monitor support and xfdesktop with improved settings dialog and better workspace support) as well as enabling GTK3 indicators for the panel in Xubuntu 13.10 by default, but no decision regarding this has been made yet.